Tin-lead alloy electroplating processes have been in use for many years, and eutectic tin-lead is used extensively in applications requiring attachment of electronic components to printed circuit boards by soldering or reflowing. During assembly, sufficient heat is applied to melt the eutectic tin-lead deposit, and upon cooling, a metallurgical bond between the component and circuit board is formed.
The temperature at which the tin-lead alloy melts is very important. Eutectic tin-lead contains 63% tin and 37% lead, and melts at a temperature of 183.degree. C. which is very well matched to the prevalent materials of construction in use today for circuit boards. At liquidus temperatures much higher than this, dimensional instability of the circuit board laminate may result. At liquidus temperatures much lower than this, the alloy may melt prematurely during prior thermal operations of the assembly process.
The electronics industry are continuously looking for alternatives to lead, as the toxic properties of this material are well known and future use may become restricted. The challenge to the industry has been finding suitable replacements for tin-lead alloy solders that possess the same or similar properties. Once found, the challenge to the electrochemist is to develop an electroplating process capable of codepositing the alloying metals in just the right proportion to impart the necessary properties to the electrodeposit.
Solid alloys of eutectic tin-silver have been used as effective solders for years, most notably in the plumbing industry where the use of lead containing solders has been prohibited because of the potential for lead leaching into the drinking water supply. Eutectic tin-silver contains 96.5% tin and 3.5% silver, and becomes liquidus at a temperature of 221.degree. C., however, as the silver content of the alloy increases slightly, the liquidus temperature increases dramatically. One hundred percent tin melts at 232.degree. C. Increasing the silver content to 10% raises the liquidus temperature to 300.degree. C., a temperature which is too high for common printed circuit board materials to withstand. Applying a temperature of less than 300.degree. C. to a tin-silver alloy containing 10% silver would not provide sufficient heat to fully melt the alloy, resulting in incomplete and inadequate formation of the solder joint. In attempting to electroplate eutectic tin-silver alloys, critical control of the silver content of the deposit is therefore essential.
W. Fluhmann, et. al., "PROPERTIES AND APPLICATION OF ELECTRODEPOSITED CU-Zn-SN ALLOYS, Am. Elect. Soc. 69.sup.th Ann. Tech. Conf. Proceedings, Vol. 2, 1982, describes a silver-tin deposit containing 90% silver and 10% tin produced from a pyrophosphate electrolyte for decorative applications, claiming a reduction in the tendency for silver to tarnish when alloyed with small amounts of tin. The wear resistance of such alloys is reportedly improved. H. Leidheiser, Jr., et. al., PULSE ELECTROPLATING OF SILVER-TIN ALLOYS AND THE FORMATION OF Ag.sub.3 Sn, J. Electrochem Soc., April 1973, pp. 484-487 describes a silver-tin electrolyte using tin stannate in place of pyrophosphate with pulsed current to improve the deposit quality. A number of other references disclose electrolytes for silver-tin alloys, including U.S. Pat. No. 5,514,261 and DE patent application 4,330,068. Here silver is the primary ingredient and the tin is present in much smaller amounts.
The electrodeposition of tin rich alloys of tin-silver is difficult given the large difference in reduction potential between the two metals. Furthermore, the preferential reduction of tin is made more difficult by the fact that silver exists in solution as a monovalent ion, whereas tin is either divalent or tetravalent and thereby requires two or four times the amount of current for reduction to occur relative to silver. In addition, an appreciable amount of silver should be present in solution to allow for the practical operation of the electrolyte on a production scale.